ENGLISH ABSTRACT: Biological invasions are a growing threat to biodiversity, trade and agriculture in South Africa.
Though alien reptiles and amphibians (herpetofauna) are not currently a major issue, escalating
problems worldwide and increased trade in South Africa suggest a possible increase in future
problems. In this thesis I explore practical measures for risk assessment implementable under
national legislation. I began by documenting record-keeping and legislative differences between
provinces in South Africa. This revealed some serious deficiencies, complicating attempts to
compile accurate inventories and discern import trends. International trade data, however,
revealed an exponential increase in the number of imports to South Africa over the last 30 years.
Characterising the abundance of species in this trade is important as species introduced in large
numbers pose a higher establishment risk. In South Africa, I found a tendency for venomous and
expensive species to be traded in low numbers, whereas species that are easy to breed and handle,
or that are colourful or patterned are traded in higher numbers.
Unlike South Africa, California and Florida have had a large number of well-documented
herpetofaunal introductions. These introductions were used to verify the role of several key
predictors in species establishment. I first evaluated the role of each variable separately. I
examined different approaches for bioclimatic modelling, the predictive power of different sources
of distribution data, and methods of assigning a climate-match score. I also present the first test of
Darwin’s naturalization hypothesis for land vertebrates using two new phylogenies inferred for
native and introduced reptiles in California and Florida. I then used boosted regression trees (BRT)
to infer the relative contribution of each factor to species establishment success. Results from the
BRTs were incorporated into a user-friendly spreadsheet model for use by assessors inexperienced in
complex modelling techniques.
Introduction effort was found to be the strongest contributor to establishment success.
Furthermore, species with short juvenile periods were more likely to establish than species that
started breeding later, as were species with more distant relatives in regional biotas. Average
climate match and life form were also important. Of the herpetofaunal groups, frogs and lizards
were most likely to establish, while snakes and turtles established at much lower rates, though
analysis of all recorded herpetofaunal introductions shows slightly different patterns. Predictions
made by the BRT model to independent data were relatively poor, though this is unlikely to be
unique to this study and can be partially explained by missing data. Though numerous uncertainties
remain in this field, many can be lessened by applying case by case rules rather than generalising
across all herpetofaunal groups. The purpose for import and potential trade volume of a species will
influence the threat it poses. Considering this in conjunction with a species’ environmental
tolerances and previous success of species with similar life histories, should provide a reasonable
and defendable estimate of establishment risk. Finally, a brief summary of the potential impacts of
introduced alien herpetofauna is provided in the thesis.
AFRIKAANSE OPSOMMING: Indringer spesies hou ‘n al groter bedreiging in vir die biodiversiteit, handel en landbou van Suid-
Afrika. Alhoewel uitheemse reptiele en amfibieërs (herpetofauna) tans nie ‘n groot bedreiging in
Suid-Afrika is nie, dui groeiende probleme wêreldwyd asook 'n toename in plaaslike handel op
moontlike toekomstige probleme. In hierdie tesis, ondersoek ek praktiese metodes vir risikobepaling
wat onder nasionale wetgewing toegepas kan word. Ek begin deur die verskille in stoor van rekords
en wetgewing tussen provinsies te dokumenteer. Hierdie proses het ernstige tekortkominge
uitgewys, wat pogings om akkurate inventarisse saam te stel en invoertendense te bepaal,
bemoeilik. Internasionale handelsdata het egter getoon dat daar ‘n eksponensiële toename in die
hoeveelheid invoere na Suid-Afrika oor die laaste 30 jaar was. Die hoeveelheid spesies in hierdie
handel is belangrik omdat spesies wat in groot hoeveelhede ingevoer word, ‘n hoër vestigingsrisiko
het. In Suid-Afrika is ‘n tendens gevind vir handel in giftige en duur spesies teen lae hoeveelhede,
terwyl spesies wat maklik teel, maklik hanteer kan word en kleurvol is of mooi patrone het, in
groter hoeveelhede mee handel gedryf word.
Kalifornië and Florida, in teenstelling met Suid-Afrika, het ‘n hoë aantal goed-gedokumenteerde
gevalle van herpetofauna wat in die natuur vrygestel is. Hierdie introduksies was gebruik om die rol
van verskeie belangrike faktore in die vestiging van populasies te bepaal. Eerstens het ek die rol van
elke faktor apart ondersoek. Ek het verskillende benaderinge vir bioklimatiese model-bou
ondersoek, die akuraatheid van verskillende bronne van distribusiedata getoets en drie metodes om
‘n “climate match score” te bereken, voorgestel. Ek bied ook die eerste toets van Darwin se
naturalisasie-hipotese vir landwerveldiere aan, deur gebruik te maak van twee nuwe filogenieë wat
ek gebou het vir inheemse en ingevoerde reptiele in Kalifornië en Florida. Ek het verder gebruik
gemaak van “boosted regression trees” (BRT) om die relatiewe bydrae van elke faktor tot die
vestigings-potensiaal van spesies te bepaal. Resultate van hierdie BRTs was ingekorporeerd in ‘n
gebruikersvriendelike ontledingstaat wat deur bestuurders, onervare in komplekse
modelboutegnieke, gebruik kan word.
Invoer-hoeveelheid was die faktor wat die sterktste bygedra het tot suksesvolle vestiging. Verder is
spesies met kort jeugperiodes en dié met verlangse familie in streeks-biotas, meer geskik om
suksesvol te vestig. Gemiddelde klimaatooreenstemming en lewensvorm was ook belangrik. Paddas
en akkedisse was die mees waarskynlikste van die herpetofauna groepe om populasies te vestig,
terwyl slange en skilpaaie teen laer tempo’s populasies gevestig het, alhoewel analise van alle
gedokumenteerde gevalle van herpetofauna-invoerings wêreldwyd effens verskillende tendense
toon. Voorspellings wat deur die BRT-model vir onafhangklike data gemaak is was redelik swak,
alhoewel hierdie resultaat onwaarskynlik nie uniek aan die studie is nie, en word gedeeltelik
verduidelik deur die gebrek aan data. Alhoewel talle onsekerhede steeds bestaan, kan dié
verminder word deur geval-tot-geval reëls toe te pas eerder as om vir herpetofauna as ‘n groep te
veralgemeen. Die doel van invoer en potensiële handel-volumes van ‘n spesie, sal die bedreiging
wat die spesie toon, bepaal. Hierdie faktore moet saam met omgewingstoleransie en voorafgaande
sukses van spesies met soortgelyke lewenswyses oorweeg word, om ‘n aanvaarbare en verdedigbare
beraming van vestigingsrisiko te gee. Laastens, word ‘n kort opsomming van die effekte wat
uitheemse herpetofauna mag hê, verskaf.
